Method for acidizing subterranean formations

ABSTRACT

Gelled acidic compositions suitable for either matrix acidizing or fracture-acidizing of subterranean formations, and methods of using said compositions, are provided. Said compositions comprise water, a water-dispersible polymer of acrylamide, an acid, a water soluble compound of a polyvalent metal wherein the metal can be reduced to a lower polyvalent valence state and cause gelation of the water containing said polymer and said acid, and a reducing agent capable of reducing said metal and causing said gelation.

This invention relates to acid treating or acidizing of subterraneanformations.

Acid treating or acidizing of porous subterranean formations penetratedby a well bore has been widely employed for increasing the production offluids, e.g., crude oil, natural gas, etc., from said formations. Theusual technique of acidizing a formation comprises introducing anon-oxidizing acid into the well under sufficient pressure to force theacid out into the formation where it reacts with the acid-solublecomponents of the formation. The technique is not limited to formationsof high acid solubility such as limestone, dolomite, etc. The techniqueis also applicable to other types of formations such as a sandstonecontaining streaks or striations of acid soluble components such as thevarious carbonates.

During the acid treating operation, passageways for fluid flow arecreated in the formation, or existing passageways therein are enlarged,thus stimulating the production of fluids from the formation. Thisaction of the acid on the formation is often called etching. Acidtreating or acidizing operations wherein the acid is injected into theformation at a pressure or rate insufficient to create cracks orfractures in the formation is usually referred to as matrix acidizing.

Hydraulic fracturing is also commonly employed to increase theproduction of fluids from subterranean formations. Hydraulic fracturingcomprises the injection of a suitable fracturing fluid down a wellpenetrating a formation and into said formation under sufficientpressure to overcome the pressure exerted by the overburden. Thisresults in creating a crack or fracture in the formation to provide apassageway which facilitates flow of fluids through the formation andinto the well. Combination fraction-acidizing processes are well knownin the art.

Thus, it is within the scope of the present invention to inject thegelled acidic compositions of the invention into the formation underinsufficient pressure to cause fracturing of the formation and carry outa matrix acidizing operation, or inject said gelled acidic compositionat sufficient rates and pressure to cause fracturing and carry out acombination fracture-acidizing operation.

One of the problems commonly encountered in acidizing operations isinsufficient penetration of the formation by the acid. It is desirablethat good penetration be obtained in order to realize maximum benefitsfrom the operation. Too often the acid is essentially completely spentin the area immediately adjacent and surrounding the well bore. Theseverity of the problem increases as the well temperature increasesbecause acid reactivity with the formation increases with increasingtemperatures, as in deeper wells.

Poor penetration can also be caused, and/or aggravated, by fluid loss tothe more porous zones of the formation where low permeability is not aproblem. Poor penetration can also be caused, and/or aggravated, byleak-off at the fracture faces in combination fracturing-acidizingoperations. Either said fluid loss or said leak-off can frequentlyworsen the situation by leaving the tight (low permeability) zones ofthe formation unchanged and merely opening up the already highpermeability zones.

One solution which has been proposed for the above discussed problem isto incorporate various polymeric thickening or viscosifying agents intothe acid solution. Said agents serve to thicken the acid solution andthus increase the viscosity thereof. It has been reported that sothickened acid solutions have reduced fluid loss properties. Forexample, see U.S. Pat. No. 3,415,319 issued in the name of B. L. Gibson;and U.S. Pat. No. 3,434,971 issued in the name of B. L. Atkins. It hasalso been reported that the reaction rate of said so-thickened acidsolutions with the acid-soluble portions of the formation is lessened orretarded. See, for example, U.S. Pat. No. 3,749,169 issued in the nameof J. F. Tate; U.S. Pat. No. 3,236,305 issued in the name of C. F.Parks; and U.S. Pat. No. 3,252,904 issued in the name of N. F.Carpenter.

Higher viscosities are also advantageous in combinationfracturing-acidizing operations in that the more viscous acidicsolutions produce wider and longer fractures. More viscous acidsolutions are also more effective in carrying propping agents into theformation when propping agents are used.

Another problem encountered in acidizing operations, particularly whenemploying acidizing compositions having thickening or viscosifyingagents incorporated therein, is stability to heat. By stability to heat,it is meant the retention of the increased or greater viscosityproperties under the conditions of use. Such compositions to besatisfactory should be sufficiently stable to resist degeneration by theheat of the formation for a period of time sufficient to accomplish theintended purpose, e.g., good penetration and significant etching of theformation. The degree of stability required in any particular operationwill vary with such operating variables as the type of formation beingtreated, the temperature of the formation, the well depth (time to pumpthe acidic composition down the well and into the formation), the acidconcentration in the composition, etc. For example, acidizing of a tightlow permeability formation will proceed more slowly than a morepermeable formation, other factors being the same, because a longer timewill be required to obtain a significant amount of etching and thecomposition must remain in place and effective for a longer period oftime. Also, more time will be required to pump the acidic compositioninto place in the formation.

The temperature of the formation usually has a pronounced effect on thestability of the acidizing compositions and, generally speaking, is oneof the most important operating variables when considering stability.Increased formation temperatures usually have at least two undesirableeffects. One such effect is degeneration of the composition, e.g.,decrease in viscosity. Another such effect is increased rate of reactionof the acid with the formation. Thus, some compositions which would besatisfactory in a low temperature formation such as in the Hugoton fieldin the Anadarko basin might not be satisfactory in formationsencountered in deeper wells as in some West Texas fields.

In ordinary acidizing operations using unthickened acids there isusually no problem in removing the spent acid because it is essentiallywater. However, a problem which is sometimes encountered when usingthickened compositions in treating formations is the ease of removal ofthe treating composition after the operation is completed. Somethickened or highly viscous solutions are difficult to remove from thepores of the formation or the fracture after the operation is complete.Sometimes a clogging residue can be left in the pores of the formation,or in the fracture. This can inhibit the production of fluids from theformation and can require costly cleanup operations. It would bedesirable to have gelled acidic compositions which break down to alesser viscosity within a short time after the operation is completed.

The present invention provides a solution for, or at least mitigates,the above discussed problems. The present invention provides improvedmethods for acidizing, or fracture-acidizing, subterranean formations;and new gelled acidic compositions for use in said methods.

Thus, in accordance with one broad aspect of the concept of theinvention, there is provided a method for acid treating a poroussubterranean formation susceptible of attack by an acid and penetratedby a well bore, which method comprises: injecting into said formationvia said well bore a gelled acidic composition comprising water; anamount of a water-dispersible polymer of acrylamide which is sufficientto thicken said water; an amount of a water-soluble compound of apolyvalent metal wherein the metal present is capable of being reducedto a lower polyvalent valence state and which is sufficient to causegelation of an aqueous dispersion of the components of said compositionwhen the valence of at least a portion of said metal is reduced to saidlower valence state; an amount of a water-soluble reducing agent whichis effective to reduce at least a portion of said metal to said lowervalence state and cause said gelation; an amount of a non-oxidizing acidwhich is capable of reacting with a significant amount of theacid-soluble components of said formation; said polymer, said polyvalentmetal compound, said reducing agent, and said acid, in the amounts used,being sufficiently compatible with each other in an aqueous dispersionthereof to permit said gelation and thus form a said composition havingsufficient stability to degeneration by the heat of said formation topermit good penetration of said composition into said formation; andmaintaining said composition in said formation in contact therewith fora period of time sufficient for the acid in said composition tosignificantly react with the acid-soluble components of said formationand stimulate the production of fluids therefrom.

Further, in accordance with another broad aspect of the concept of theinvention there is provided a gelled acidic composition, suitable formatrix acidizing or fracture-acidizing of a subterranean formation,comprising: water; a water-thickening amount of a water-dispersiblepolymer of acrylamide; an amount of a water-soluble compound of apolyvalent metal wherein the metal present is capable of being reducedto a lower polyvalent valence state and which is sufficient to causegelation of an aqueous dispersion of the components of said compositionwhen the valence of at least a portion of said metal is reduced to saidlower valence state; an amount of a water-soluble reducing agent whichis effective to reduce at least a portion of said metal to said lowervalence state and cause said gelation; and an amount of a non-oxidizingacid which is capable of reacting with a significant amount of theacid-soluble components of said formation; said polymer, said polyvalentmetal compound, said reducing agent, and said acid, in the amounts used,being sufficiently compatible with each other in an aqueous dispersionthereof to permit said gelation and thus form a said composition havingsufficient stability to degeneration by the heat of said formation topermit good penetration of said composition into said formation and themaintenance of said composition in said formation in contact therewithfor a period of time sufficient for the acid in said composition tosignificantly react with the acid-soluble components of said formationand stimulate the production of fluids therefrom.

As noted above, the gelled acidic compositions of the invention must besuitable for matrix acidizing or fracture-acidizing of subterraneanformations. In order to satisfy this requirement, the polymer, thepolyvalent metal compound, the reducing agent, and the acid, in theamounts used, must be sufficiently compatible with each other, in anaqueous dispersion thereof, to permit the gelation of said dispersionand thus form a said composition having sufficient stability todegeneration by the heat of the formation to permit good penetration ofsaid composition into the formation. Furthermore, once said penetrationhas been attained, the said stability must be sufficient to permit themaintaining of said composition in contact with the formation for aperiod of time sufficient for the acid in the composition tosignificantly react with the acid-soluble components of the formationand stimulate the production of fluids therefrom, e.g., by creating newpassageways or enlarging existing passageways through said formation.

Herein and in the claims, unless otherwise specified, the term "goodpenetration" means penetration of live or effective acid into theformation a sufficient distance to result in stimulating the productionof fluids therefrom, e.g., by the creation of sufficient newpassageways, or sufficient enlargement of existing passageways, throughsaid formation to significantly increase the production of fluids fromthe formation. This can vary for different formations, well spacings,and what it is desired to accomplish in a given acidizing treatment.Those skilled in the art will usually know what will be "goodpenetration" for a given formation and a given type of treatment.However, generally speaking, for guidance purposes in the practice ofthe invention and not by way of limitation of the invention, "goodpenetration" will usually be considered to be a distance of a few feet,e.g., up to 5 or more, in a small volume matrix acidizing operation, andseveral hundred feet, e.g., up to 500 or more, in a large volumefracture-acidizing operation.

Herein and in the claims, unless otherwise specified, the term "polymer"is employed generically to include both homopolymers and copolymers; andthe term "water-dispersible polymers" is employed generically to includethose polymers which are truly water-soluble and those polymers whichare dispersible in water or other aqueous medium to form stablecolloidal suspensions which can be gelled as described herein. Also, theterm "aqueous dispersion" is employed generically to include both truesolutions and stable colloidal suspensions of the components of thecompositions of the invention which can be gelled as described herein.

Any suitable polymer of acrylamide meeting the above statedcompatibility requirements can be used in the practice of the invention.Thus, under proper conditions of use, such polymers can include variouspolyacrylamides and related polymers which are water-dispersible andwhich can be used in an aqueous medium, with the gelling agentsdescribed herein, to give an aqueous gel. These can include the varioussubstantially linear homopolymers and copolymers of acrylamide andmethacrylamide. By substantially linear it is meant that the polymersare substantially free of crosslinking between the polymer chains. Saidpolymers can have up to about 75, preferably up to about 45, percent ofthe carboxamide groups hydrolyzed to carboxyl groups. One presentlypreferred group of polymers includes those wherein from about 20 toabout 25 percent of the carboxamide groups are hydrolyzed. As usedherein and in the claims, unless otherwise specified, the term"hydrolyzed" includes modified polymers wherein the carboxyl groups arein the acid form and also such polymers wherein the carboxyl groups arein the salt form, provided said salts are water-dispersible. Such saltsinclude the ammonium salts, the alkali metal salts, and others which arewater-dispersible. Hydrolysis can be carried out in any suitablefashion, for example, by heating an aqueous solution of the polymer witha suitable amount of sodium hydroxide.

Substantially linear polyacrylamides can be prepared by methods known inthe art. For example, the polymerization can be carried out in aqueousmedium, in the presence of a small but effective amount of awater-soluble oxygen-containing catalyst, e.g., a thiosulfate orbisulfate of potassium or sodium or an organic hydroperoxide, at atemperature between about 30° and 80° C. The resulting polymer isrecovered from the aqueous medium, as by drum drying, and can besubsequently ground to the desired particle size. The particle sizeshould be fine enough to facilitate dispersion of the polymer in water.A presently preferred particle size is such that about 90 weight percentwill pass through a number 10 mesh sieve, and not more than about 10weight percent will be retained on a 200 mesh sieve (U.S. Bureau ofStandards Sieve Series).

Under proper conditions of use, examples of copolymers which can be usedin the practice of the invention can include the water-dispersiblecopolymers resulting from the polymerization of a major proportion ofacrylamide or methacrylamide and a minor proportion of an ethylenicallyunsaturated monomer copolymerizable therewith. It is desirable thatsufficient acrylamide or methacrylamide be present in the monomersmixture to impart to the copolymer the above-described water-dispersibleproperties, for example, from about 60 to 99 percent acrylamide and fromabout 1 to 40 percent other ethylenically unsaturated monomers,preferably from about 75 to about 95 percent acrylamide and from about 5to about 25 percent other ethylenically unsaturated monomer. Such othermonomers include acrylic acid, methacrylic acid, vinylsulfonic acid,vinylbenzylsulfonic acid, vinylbenzenesulfonic acid, vinyl acetate,acrylonitrile, methyl acrylonitrile, vinyl alkyl ether, vinyl chloride,maleic anhydride, vinyl substituted cationic quaternary ammoniumcompounds, and the like. Various methods are known in the art forpreparing said copolymers. For example, see U.S. Pat. Nos. 2,625,529;2,740,522; 2,729,557; 2,831,841; and 2,909,508. Said copolymers can alsobe used in the hydrolyzed form, as discussed above for the homopolymers.

Crosslinked polyacrylamides and crosslinked polymethacrylamides, atvarious stages of hydrolysis as described above, and meeting theabove-stated compatibility requirements, can also be used in thepractice of the invention. In general, said crosslinked polyacrylamidescan be prepared by the methods described above, but including in themonomeric mixture a suitable amount of a suitable crosslinking agent.Examples of crosslinking agents can include methylenebisacrylamide,divinylbenzene, vinyl ether, divinyl ether, and the like. Saidcrosslinking agents can be used in small amounts, e.g., up to about 1percent by weight of the monomeric mixture. Such crosslinking is to bedistinguished from any crosslinking which occurs when solutions ofpolymers and the other components of the gelled acidic compositions ofthe invention are gelled as described herein.

All the polymers useful in the practice of the invention arecharacterized by high molecular weight. The molecular weight is notcritical so long as the polymer has the above-describedwater-dispersible properties. It is preferred that the polymer have amolecular weight of at least 500,000, more preferably at least about2,000,000. The upper limit of molecular weight is unimportant so long asthe polymer is water-dispersible, and the gelled acidic compositiontherefrom can be pumped. Thus, it is within the scope of the inventionto use polymers having molecular weights as high as 20,000,000 orhigher, and meeting said conditions.

The amount of the above-described polymers used in preparing the gelledacidic compositions of the invention can vary widely depending upon theparticular polymer used, the purity of said polymer, and propertiesdesired in said compositions. In general, the amount of polymer usedwill be a water-thickening amount, i.e., at least an amount which willsignificantly thicken the water to which it is added. For example,amounts in the order of 25 to 100 parts per million by weight (0.0025 to0.01 weight percent) have been found to significantly thicken water.Distilled water containing 25 ppm of a polymer of acrylamide having amolecular weight of about 10 × 10⁶ had a viscosity increase of about 41percent. At 50 ppm the viscosity increase was about 106 percent. At 100ppm the viscosity increase was about 347 percent. As another example,distilled water containing 25 ppm of a polymer of acrylamide having amolecular weight of about 3.5 × 10⁶ had a viscosity increase of about 23percent. At 50 ppm the viscosity increase was about 82 percent. At 100ppm the viscosity increase was about 241 percent. Generally speaking,amounts of the above-described polymers in the range of from 0.01 to 4,preferably from 0.1 to 1.5, more preferably 0.1 to 0.5, weight percent,based on the total weight of the composition, can be used in preparinggelled acidic compositions for use in the practice of the invention.However, amounts outside said ranges can be used. In general, with theproper amounts of polyvalent metal and reducing agent, the amount ofpolymer used will determine the consistency of the gel obtained. Smallamounts of polymer will usually produce liquid mobile gels which can bereadily pumped. Large amounts of polymer will usually produce thicker,more viscous, somewhat elastic gels.

Metal compounds which can be used in the practice of the invention arewater-soluble compounds of polyvalent metals wherein the metal ispresent in a valence state which is capable of being reduced to a lowerpolyvalent valence state, and which will meet the above-statedcompatibility requirements. Thus, under proper conditions of use,examples of such compounds can include potassium permanganate, sodiumpermanganate, ammonium chromate, ammonium dichromate, the alkali metalchromates, the alkali metal dichromates, and chromium trioxide. Sodiumdichromate and potassium dichromate, because of low cost and readyavailability, are the presently preferred metal-containing compounds.The hexavalent chromium in said chromium compounds is reduced in situ totrivalent chromium by suitable reducing agents, as discussedhereinafter. In the permanganate compounds the manganese is reduced from+7 valence to +4 valence as in MnO₂.

The amount of said metal-containing compounds used will be a small butfinite amount which is effective or sufficient to cause gelation of anaqueous dispersion of the starting components of the compositions of theinvention when the metal in the polyvalent metal compound is reduced toa lower polyvalent valence state. The lower limit of the concentrationof the starting metal-containing compound will depend upon severalfactors including the particular type of polymer used, the concentrationof the polymer, and the type of gel product desired. For similarreasons, the upper limit on the concentration of the startingmetal-containing compound also cannot always be precisely defined.However, it should be noted that excessive amounts of the starting metalcompound, for example +6 chromium, which can lead to excessive amountsof +3 chromium when there is sufficient reducing agent present to reducethe excess +6 chromium, can adversely affect the stability of the gelledcompositions. It is believed this can provide one valuable method forcontrolling stability or life span so as to obtain gelled acidiccompositions which will break down with time and/or temperature topermit ready well clean-up subsequent to an acidizingfracturing-acidizing operation. As a general guide, the amount of thestarting polyvalent metal-containing compound used in preparing thegelled acidic compositions of the invention will be in the range of from0.05 to 30, preferably 0.5 to 20, weight percent of the amount of thepolymer used. However, in some situations it may be desirable to useamounts of the starting polyvalent metal-containing compound which areoutside the above ranges. Such use is within the scope of the invention.Those skilled in the art can determine the amount of starting polyvalentmetal-containing compound to be used by suitable experiments carried outin the light of this disclosure.

Suitable water-soluble reducing agents which can be used in the practiceof the invention are those meeting the above-stated compatibilityrequirements. Under proper conditions of use this can includesulfur-containing compounds such as sodium sulfite, potassium sulfite,sodium hydrosulfite, potassium hydrosulfite, sodium metabisulfite,potassium metabisulfite, sodium bisulfite, potassium bisulfite, sodiumsulfide, potassium sulfide, sodium thiosulfate, potassium thiosulfate,ferrous sulfate, thioacetamide, hydrogen sulfide, and others; andnonsulfur-containing compounds such as hydroquinone, ferrous chloride,p-hydrazinobenzoic acid, hydrazine phosphite, hydrazine dichloride, andothers. Some of the above reducing agents act more quickly than others.For example, sodium bisulfite has been found to cause extremely rapidgelation with the higher concentrations of polymer.

One presently preferred group of reducing agents are the water-solubleorganic compounds containing from 1 to about 10 carbon atoms permolecule and which release hydrogen sulfide upon hydrolysis. Thesecompounds contain the group ═C═S and include organic amides, xanthatesalts, trithiocarbonate salts, and dithiocarbamate salts. Some examplesare: thioacetamide, thiourea, thioformamide, thiopropionamide, sodiumethyl xanthate, N,N-diethyl sodium dithiocarbamate, sodiumbutyltrithiocarbonate, and the like. Mixtures of said reducing agentscan also be used.

The amount of reducing agent to be used in preparing the gelled acidiccompositions of the invention will be a small but finite amount which iseffective or sufficient to reduce at least a portion of the highervalence metal in the starting polyvalent metal-containing compound to alower polyvalent valence state. Thus, the amount of reducing agent to beused depends, to some extent at least, upon the amount of the startingpolyvalent metal-containing compound which is used. In many instances,it will be preferred to use an excess of reducing agent to compensatefor dissolved oxygen in the water, exposure to air during preparation ofthe gels, and possible contact with other oxidizing substances such asmight be encountered in field operations. As a general guide, the amountof reducing agent used will generally be within the range of from 0.1 toat least 150, preferably at least about 200, weight percent of thestoichiometric amount required to reduce the metal in the startingpolyvalent metal compound to said lower polyvalent state, e.g., +6 Cr to+3 Cr. In most instances it will be preferred to use at least astoichiometric amount. However, in some instances, it may be desirableto use amounts of reducing agent outside said ranges. The use of suchamounts is within the scope of the invention. Those skilled in the artcan determine the amount of reducing agent to be used by suitable simpleexperiments carried out in the light of this disclosure.

Acids useful in the practice of the invention include any non-oxidizingacid meeting the above-stated compatibility requirements and which iseffective in increasing the flow of fluids, e.g., hydrocarbons, throughthe formation and into the well. Thus, under proper conditions of use,examples of such acids can include inorganic acids such as hydrochloricacid, and sulfuric acid; C₁ -C₃ organic acids such as formic acid,acetic acid, propionic acid, and mixtures thereof, and combinations ofinorganic and organic acids. The concentration or strength of the acidcan vary depending upon the type of acid, the type of formation beingtreated, the above-stated compatibility requirements, and the resultsdesired in the particular treating operation. The concentration can varyfrom about 1 to about 60 weight percent, with concentrations within therange of 5 to 50 weight percent usually preferred, based upon the totalweight of the gelled acidic composition. When an inorganic acid such ashydrochloric acid is used it is presently preferred to use an amountwhich is sufficient to provide an amount of HCl within the range of from1 to 12, more preferably up to about 10, weight percent based on thetotal weight of the composition. The acids used in the practice of theinvention can contain any of the known corrosion inhibitors,deemulsifying agents, sequestering agents, surfactants, frictionreducers, etc., known in the art, and which meet the above-statedcompatibility requirements.

The gelled acidic compositions of the invention are aqueouscompositions. They normally contain a significant amount of water. Theamount of said water can vary widely depending upon the concentrationsof the other components in the compositions, particularly theconcentration of the acid. For example, when an organic acid such asacetic acid is used in the maximum concentration of 60 weight percentthe amount of water present in the composition clearly will be less thanwhen an inorganic acid such as HCl is used in the preferred maximumconcentration of about 10 weight percent. Thus, while no precise overallrange of water content can be set forth, based on the above-statedoverall ranges for the concentrations of said other components the watercontent of said compositions can be in the range of from about 5 toabout 99, frequently about 50 to about 95, weight percent. However,amounts of water outside said ranges can be used.

Propping agents can be included in the gelled acidic compositions of theinvention if desired. Propping agents which can be used include any ofthose known in the art, e.g., sand grains, walnut shell fragments,tempered glass beads, aluminum pellets, and similar materials, so longas they meet the above-stated compatibility requirements. Generallyspeaking, it is desirable to use propping agents having particle sizesin the range of 8 to 40 mesh (U.S. Sieve Series). However, particlesizes outside this range can be employed. When propping agents are usedthey should be made of materials which are not severely attacked by theacid used during the time they are exposed to said acid.

Any suitable method can be employed for preparing the gelled acidiccompositions of the invention. Thus, any suitable mixing technique ororder of addition of the components of said composition to each othercan be employed which will provide a said composition having sufficientstability to degeneration by the heat of the formation (in which thecomposition is to be used) to permit good penetration of the compositioninto, and significant etching of, said formation. However, it isordinarily preferred to first dissolve or disperse the polymer in waterbefore contacting the polymer with acid. Thus, it is preferred to avoidcontacting the dry polymer with aqueous acid. Some suitable mixingorders, with the components named in order of mixing, include:water--polymer--polyvalent metal compound--reducing agent--acid;water--polymer--acid--polyvalent metal compound--reducing agent; andwater--polymer--polyvalent metal compound--acid--reducing agent; and thelike. It is within the scope of the invention to moisten or slurry thepolymer with a small amount, e.g., about 1 to about 6 weight percentbased on the weight of the polymer, of a low molecular weight alcohol,e.g., C₁ to C.sub. 3 alcohols, as a dispersion aid, prior to dispersingthe polymer in water. Contact of the polyvalent metal compound andreducing agent in the absence of the dispersed polymer should beavoided. Since the acid may sometimes have a degrading effect on thepolymer, it is preferred that the acid not be in contact with thepolymer, even in aqueous solution, unduly long in the absence of thegelling agents. Similarly, it is preferred that there be no undue delaybetween completing the preparation of the gelled acidic composition andits introduction into contact with the formation.

As used herein and in the claims, unless otherwise specified, the statedvalues for "degree of hydrolysis" or "percent hydrolyzed", and liketerms, refer to initial values prior to use or test of the polymer.Unless otherwise stated, said values were obtained by the followinganalytical procedure. Place 200 ml of distilled water in a beakerprovided with a magnetic stirrer. Weigh a 0.1 gram polymer sampleaccurately to ±0.1 mg. Start the stirrer and quantitatively transfer theweighed sample into the water vortex. Stir at a rapid rate overnight.Using a pH meter and 1:1 HCl, adjust the pH of the sample solution toless than 3.0. Stir the solution for 30 minutes. Adjust the pH of thesolution to exactly 3.3 by dropwise addition of 0.1 N NaOH. Then slowlytitrate with standard 0.1 N NaOH from pH 3.3 to pH 7.0.

    % hydrolysis = (V × N × 0.072 × 100)/W

where: V = ml of base used in titration; N = normality of base; W =grams of polymer sample; and 0.072 = milliequivalent weight of acrylicacid.

The gelled acidic compositions of the invention can be prepared on thesurface in a suitable tank equipped with suitable mixing means, and thenpumped down the well and into the formation employing conventionalequipment for pumping acidic compositions. However, it is within thescope of the invention to prepare said compositions while they are beingpumped down the well. This technique is sometimes referred to as "on thefly". For example, a solution of the polymer in water can be prepared ina tank adjacent the well head. Pumping of this solution through aconduit to the well head can then be started. Then, a few feetdownstream from the tank a suitable connection can be provided forintroducing the polyvalent metal compound into said conduit, either drythrough a mixing hopper, or preferably as an aqueous solution. Then, afew feet farther downstream the reducing agent can be similarlyintroduced, preferably as an aqueous solution. The acid can then beintroduced into said conduit a few feet downstream from the reducingagent. As will be understood by those skilled in the art, the rate ofintroduction of said components into said conduit will depend upon thepumping rate of the polymer solution through said conduit. Any of theabove-mentioned orders of addition can be employed in said "on the fly"technique. Mixing orifices can be provided in said conduit, if desired.

It is within the scope of the invention to precede the injection of thegelled acidic composition into the well and out into the formation witha preflush of a suitable cooling fluid, e.g., water. Such fluids serveto cool the well tubing and formation and extend the useful operatingtemperature range of said compositions. The volume of said cooling fluidso injected can be any suitable volume sufficient to significantlydecrease the temperature of the formation being treated, and can varydepending upon the characteristics of the formation. For example,amounts up to 20,000 gallons, or more, can be used to obtain atemperature decrease in the order of 100° to 250° F.

The following examples will serve to further illustrate the invention,but should not be considered as unduly limiting on the invention.

EXAMPLE I

A 15g quantity of an acrylamide polymer (Dow Pusher 700) having amolecular weight of about 5,500,000 and a degree of hydrolysis of about23.5% was blended into 500 ml of tap water with the aid of a high speedmixer (Hamilton Beach malt mixer) for one minute. After standing at roomtemperature for 5 days, a 100 ml portion of this 3 weight percentpolymer solution was transferred to a pint jar. To this was added 2.5 mlof a 10 weight percent solution of sodium dichromate dihydrate withstirring followed by the addition of 0.3 g thioacetamide. About 1 minutelater, 100 ml glacial acetic acid was added and the mixture was stirredfor 1 minute more. All the reagents and acid blended quite well.

A portion of this composition (50% acetic acid, 1.5% polymer, 1250 ppmNa₂ Cr₂ O₇ ·2H₂ O, 1500 ppm thioacetamide, by weight) was transferred toa capillary viscometer (Kimax No. 500) and the viscometer was placed ina water bath at about 85° F. The temperature of the bath was thenincreased at a rate sufficient to reach 200° F. in about 1 hour. Theefflux time of the composition was measured at intervals and recordedalong with the time and temperature.

For purposes of comparison, another similar composition was preparedsimilarly, but omitting the dichromate and thioacetamide crosslinkingagents. The essential conditions and results of these tests are shown inTable I below.

                  TABLE I                                                         ______________________________________                                                   Gelled     Non-Gelled                                                           Bath     Efflux  Bath   Efflux                                                Temp.    Time    Temp.  Time                                     Time in Bath (min)                                                                         (° F)                                                                           (sec)   (° F)                                                                         (sec)                                    ______________________________________                                        0            83       59.2    81     71.0                                     5            88       63.7    87     42.0                                     10           98       63.5    101    35.0                                     15           110      69.0    115    31.0                                     20           123      82.2    127    27.4                                     25           139      92.5    139    22.5                                     30           149      85      151    21.0                                     35           161      77.8    162    20.8                                     40           172      68.0    171    18.1                                     45           179      61.6    180    16.4                                     50           185      38.7    185    15.1                                     55           191      36.8    191    14.3                                     60           194      27.3    195    12.6                                     65           198      26.0    201    12.0                                     70           201      40.9    --     --                                       ______________________________________                                    

The data in Table I show that, with increasing time and increasingtemperature, the non-gelled acid system exhibited a relatively lowviscosity (short efflux time) which continuously decreased throughoutthe test period. In contrast, the gelled acid system of the inventionexhibited a significantly greater viscosity (longer efflux time)throughout the test period. Moreover, the viscosity was seen to increaseup to about 139° F at which point the material appeared to gel. Thecomposition also appeared to re-gel at 201° F as evidenced by anincrease in viscosity.

It should be noted that both compositions were more viscous than watereven at the completion of the test. For comparison, pure water wouldexhibit an efflux time of one second or less with the viscometer usedabove.

Based on the above data, it is concluded that the gelled acidiccomposition comprising a solution of a substantially linearpolyacrylamide having a degree of hydrolysis of about 23.5% and havingincorporated therein sodium dichromate dihydrate, thioacetamide reducingagent, and acetic acid is a preferred composition in accordance with theinvention. From the above viscosity data it is concluded that because ofits greater viscosity the gelled composition of the invention would bemarkedly superior to the ungelled composition, particularly infracture-acidizing operations. From said viscosity data, it is furtherconcluded that the components of the gelled composition have sufficientcompatibility with each other to permit good penetration (as definedabove) into the formation, and permit maintaining of the composition incontact with the formation for a period of time usually sufficient forthe acid to significantly react with the acid-soluble components of theformation. Thus, it is further concluded that suitable compositions inaccordance with the invention could be used advantageously for acidizingoperations in wells having a depth of up to at least 10,000 feet, and atformation temperatures of up to at least 200° F. The use of a preflushcooling fluid injected down the well and into the formation prior to theinjection of the gelled composition would extend said ranges ofoperation. As will be understood by those skilled in the art, the actualattainable ranges of effective acidizing operation will depend upon theviscosity of the gelled composition, the formation temperature, thecomposition of the formation, the rate of injection of the gelled acidiccomposition, the acid concentration in said gelled acidic composition,etc.

While certain embodiments of the invention have been described forillustrative purposes, the invention is not limited thereto. Variousother modifications or embodiments of the invention will be apparent tothose skilled in the art in view of this disclosure. Such modificationsor embodiments are within the spirit and scope of the disclosure.

We claim:
 1. A method for acid treating a porous subterranean formationsusceptible of attack by an acid and penetrated by a well bore, whichmethod comprises:injecting into said formation via said well bore agelled acidic composition comprising water; an amount of awater-dispersible polymer of acrylamide which is sufficient to thickensaid water; an amount of a water-soluble compound of a polyvalent metalwherein the metal present is capable of being reduced to a lowerpolyvalent valence state and which is sufficient to cause gelation of anaqueous dispersion of the components of said composition when thevalence of at least a portion of said metal is reduced to said lowervalence state; an amount of a water-soluble reducing agent which iseffective to reduce at least a portion of said metal to said lowervalence state and cause said gelation; an amount of a non-oxidizing acidwhich is capable of reacting with a significant amount of theacid-soluble components of said formation; said polymer, said polyvalentmetal compound, said reducing agent, and said acid, in the amounts used,being sufficiently compatible with each other in an aqueous dispersionthereof to permit said gelation and thus form a said composition havingsufficient stability to degeneration by the heat of said formation topermit good penetration of said composition into said formation; andmaintaining said composition in said formation in contact therewith fora period of time usually sufficient for the acid in said composition tosignificantly react with the acid-soluble components of said formationand stimulate the production of fluids therefrom.
 2. A method accordingto claim 1 wherein:the amount of said polymer is within the range offrom 0.01 to about 4 weight percent based upon the total weight of saidcomposition; the amount of said polyvalent metal compound is within therange of from 0.05 to 30 weight percent based upon the weight of saidpolymer; the amount of said reducing agent is within the range of from0.1 to about 200 percent of the stoichiometric amount required to reducesaid polyvalent metal to said lower valence state; and the amount ofsaid acid is within the range of from 1 to 60 weight percent, based onthe total weight of said composition.
 3. A method according to claim 2wherein:said polymer is a polyacrylamide having a molecular weight of atleast about 2 million and wherein from about 20 to about 25 percent ofthe amide groups therein have been hydrolyzed to carboxyl groups; andsaid acid is an organic acid selected from the group consisting offormic acid, acetic acid, propionic acid, and mixtures thereof.
 4. Amethod according to claim 3 wherein:said polyvalent metal compound issodium dichromate or potassium dichromate; said acid is acetic acid; andsaid reducing agent is thioacetamide.
 5. A method according to claim 1wherein said polymer is a polyacrylamide.
 6. A method according to claim1 wherein said acid is an organic acid selected from the groupconsisting of formic acid, acetic acid, propionic acid, and mixturesthereof.
 7. A method according to claim 1 wherein said gelled acidiccomposition is injected into said formation at a pressure sufficient tofracture said formation.
 8. A method for acid treating a poroussubterranean formation susceptible of attack by an acid and penetratedby a well bore, which method comprises:injecting into said formation viasaid well bore a gelled acidic composition comprising water; an amountof a water-dispersible polymer of acrylamide which is sufficient tothicken said water; an amount of a water-soluble compound of apolyvalent metal wherein the metal present is capable of being reducedto a lower polyvalent valence state and which is sufficient to causegelation of an aqueous dispersion of the components of said compositionwhen the valence of at least a portion of said metal is reduced to saidlower valence state; an amount of a water-soluble reducing agent whichis effective to reduce at least a portion of said metal to said lowervalence state and cause said gelation; an amount of a non-oxidizing acidwhich is capable of reacting with a significant amount of theacid-soluble components of said formation; said polymer, said polyvalentmetal compound, said reducing agent, and said acid, in the amounts used,being sufficiently compatible with each other in an aqueous dispersionthereof to permit said gelation and thus form a said composition havingsufficient stability to degeneration by the heat of said formation topermit good penetration of said composition into said formation; andmaintaining said composition in said formation in contact therewith fora period of time usually sufficient for the acid in said composition tosignificantly react with the acid-soluble components of said formationand stimulate the production of fluids therefrom; said gelled acidiccomposition being prepared and injected into said formation by thefollowing combination of steps: a. dispersing said polymer is saidwater; b. pumping said water containing said polymer into said well viaa suitable conduit; c. then introducing one of said polyvalent metalcompound component and said acid component into said conduit during saidpumping; d. then during said pumping, introducing the other of saidpolyvalent metal compound component and said acid component, which wasnot introduced in said step (c), into said conduit downstream from thepoint of introduction in said step (c); and e. then during said pumping;introducing said reducing agent into said conduit downstream from thepoint of introduction in said step (d).
 9. A method according to claim 8wherein the amount of said polymer is within the range of from 0.1 toabout 1.5 weight percent based upon the total weight of saidcomposition.
 10. A method according to claim 8 wherein, prior to saidstep (b), a preflush of a cooling fluid is injected into said formationin an amount sufficient to significantly decrease the temperature ofsaid formation.
 11. A method according to claim 8 wherein:the amount ofsaid polymer is within the range of from 0.01 to about 4 weight percentbased upon the total weight of said composition; the amount of saidpolyvalent metal compound is within the range of from 0.05 to 30 weightpercent based upon the weight of said polymer; the amount of saidreducing agent is within the range of from 0.1 to about 200 percent ofthe stoichiometric amount required to reduce said polyvalent metal tosaid lower valence state; and the amount of said acid is within therange of from 1 to about 60 weight percent, based on the total weight ofsaid composition.
 12. A method according to claim 11 wherein:saidpolymer is a polyacrylamide having a molecular weight of at least about2 million and wherein from about 20 to about 25 percent of the amidegroups therein have been hydrolyzed to carboxyl groups; and said acid isan organic acid selected from the group consisting of formic acid,acetic acid, propionic acid, and mixtures thereof.
 13. A methodaccording to claim 12 wherein:the amount of said polymer is within therange of from about 0.1 to about 1.5 weight percent; said polyvalentmetal compound is sodium dichromate or potassium dichromate; said acidis acetic acid; and said reducing agent is thioacetamide.
 14. A methodaccording to claim 8 wherein said polymer is a polyacrylamide.
 15. Amethod according to claim 8 wherein said acid is an organic acidselected from the group consisting of formic acid, acetic acid,propionic acid, and mixtures thereof.
 16. A method according to claim 8wherein said gelled acidic composition is injected into said formationat a pressure sufficient to fracture said formation.
 17. A method foracid treating a porous subterranean formation susceptible of attack byan acid and penetrated by a well bore, which method comprises:injectinginto said formation via said well bore a gelled acidic compositioncomprising water; an amount of a water-dispersible polymer of acrylamidewhich is sufficient to thicken said water; an amount of a water-solublecompound of a polyvalent metal wherein the metal present is capable ofbeing reduced to a lower polyvalent valence state and which issufficient to cause gelation of an aqueous dispersion of the componentsof said composition when the valence of at least a portion of said metalis reduced to said lower valence state; an amount of a water-solublereducing agent which is effective to reduce at least a portion of saidmetal to said lower valence state and cause said gelation; an amount ofa non-oxidizing acid which is capable of reacting with a significantamount of the acid-soluble components of said formation; said polymer,said polyvalent metal compound, said reducing agent, and said acid, inthe amounts used, being sufficiently compatible with each other in anaqueous dispersion thereof to permit said gelation and thus form a saidcomposition having sufficient stability to degeneration by the heat ofsaid formation to permit good penetration of said composition into saidformation; and maintaining said composition in said formation in contacttherewith for a period of time usually sufficient for the acid in saidcomposition to significantly react with the acid-soluble components ofsaid formation and stimulate the production of fluids therefrom; saidgelled acidic composition being prepared and injected into saidformation by the following combination of steps: a. dispersing saidpolymer in said water; b. then initiating pumping of said watercontaining said polymer into said well via a suitable conduit; c. thenintroducing said polyvalent metal compound into said conduit; d. thenintroducing said reducing agent into said conduit downstream from thepoint of introduction of said polyvalent metal compound; and e. thenintroducing said acid into said conduit downstream from the point ofintroduction of said reducing agent.
 18. A method according to claim 17wherein the amount of said polymer is within the range of from 0.1 to1.5 weight percent.
 19. A method according to claim 17 wherein, prior tosaid step (b), a preflush of a cooling fluid is injected into saidformation in an amount sufficient to significantly decrease thetemperature of said formation.